The readability of the information displayed by digital display devices such as liquid crystal display cells is determined, in particular, by the surface area offered by the digital display device for displaying information. For a given display surface area, a compromise must therefore be found between the amount and size of information that is required to be displayed. The larger the size of the displayed information is, the better the readability of the information will be. However, the amount of information that it will be possible to display will necessarily be limited. Conversely, if the size of the displayed information is reduced, the amount of information can be increased, but this will be at the expense of readability.
To overcome this drawback, it has already been proposed to arrange a second display device underneath a first display device. Information can therefore be displayed partly with the first display device, and partly with the second display device. By switching between the first and second display device, it is possible to choose to reveal the information displayed by the first or second display device, which makes it possible to display a larger amount of information and in a larger size.
The drawback of such a display assembly including two superposed display devices is its electrical power consumption which is quite high. Such display assemblies are usually intended to be mounted in portable objects of small dimensions, such as wristwatches, whose power reserves are limited.
Further, the readability of the information displayed by display devices such as liquid crystal display cells or organic light emitting diode display devices is very dependent on ambient lighting conditions. With some display devices, the displayed information can be read in good conditions in a lit environment, but is difficult to read in a dark environment. Conversely, other categories of display devices provide a good quality display in twilight or darkness, but are difficult to read in broad daylight.
By way of example, let us consider transflective liquid crystal display cells, that is to say liquid crystal display cells capable of displaying information that will be visible in daytime by means of a reflection phenomenon, and which will also be visible at night by transmission by using a backlighting device. Such transflective liquid crystal display cells are optimised to provide the best possible reflection of sunlight and thus to ensure good readability of the displayed information in bright ambient conditions. However, in order for such transflective liquid crystal display cells to be capable of the best possible reflection of sunlight, their transmission efficiency is greatly restricted. Thus, when the backlighting device is activated to allow the displayed information to be read in twilight, most of the light emitted by the backlighting device is lost in absorption phenomena. Energy efficiency is therefore poor. Further, the optical qualities of the information displayed by the liquid crystal cell are greatly dependent on the viewing angle.
As regards emissive display devices, such as organic light-emitting diode display devices, these devices have superior optical qualities to those of liquid crystal display cells, since the optical qualities are not dependent on the viewing angle. Nonetheless, these high quality emissive display devices do not permit a reflective mode of operation. The information displayed thereby is thus very readable in twilight or darkness, but becomes difficult to read once observed outdoors. To overcome this problem, it is possible to increase the amount of current supplied to emissive display devices in order to ensure a minimum level of readability. However, even in normal conditions of use, these emissive display devices use more power than a reflective liquid crystal cell. Their electrical power consumption is such that it is difficult to envisage keeping them permanently on, in particular when they are incorporated in a portable object of small dimensions, such as a wristwatch, whose only source of energy is a battery which is usually required to last for more than one year.